1. Field of the Invention
The invention relates to an electronic circuit for a surveillance of a final amplifier and its load and in particular of such final amplifier in connection with an anti-skid system for vehicles.
2. Brief Description of the Background of the Invention Including Prior Art
Anti-skid systems in motor vehicles set particular high standards and requirements relating to their functional reliability and safety. All possibly occurring electrical or mechanical defects have to be recognized as quickly as possible. The electronic circuit then has to react in such a way that the system, for example by way of a partial shutoff, is returned into a safe state. In extreme cases, the anti-skid protection has to be turned off such that the vehicle can then be braked only in a conventional manner.
Test circuits for the electronic circuitry of anti-skid systems are known For example, the German Patent Application Laid Out DE-A-2,631,569 teaches that test pulses are fed into a control channel, which test pulses are scanned behind the final amplifier and which are investigated relative to their proper values The pulses are in this case of such a short duration that the following solenoid valve does not switch mechanically.
A part system of the anti-skid system to be monitored in particular is the final stage of the anti-skid system. Conventionally, this final stage is controlled by a logic circuit or by a microcomputer corresponding to the wheel speeds or, respectively, the wheel accelerations of the monitored wheel.
The power transistor of the final stage controls the power supply of a solenoid valve for controlling the corresponding wheel brake.
The following errors can occur in this context. The winding of the solenoid valve, representing a load, can experience an interruption, a short circuit, or an impermissible resistance value. The same error possibilities exist also for the connection cable between the final stage and the load.
Furthermore, the final transistor itself can exhibit an open circuit or a short circuit connection.
In order to sense the errors of the recited kind, it is conceivable to include a low-impedance current measuring resistor in the connection line to the load. With such a low-impedance current measuring resistor, a current value, which is either too high or too low, can be recognized. In case of the high currents to be switched in an anti-skid system, the voltage drop at the measuring resistor would, however, be impermissibly high.
It is further conceivable to recognize an error indirectly via a switch-on-duration surveillance device monitoring the solenoid valve usually present in the system. However, this is not possible in all cases. For example, in case of a down-shifting in a vehicle on a slippery road, covered with ice or snow, a wheel can lock and skid during an extended time period without that a braking occurs. An error would then be indicated in this case, even though the circuit were in fact to operate correctly.
The German Patent DE-2,701,159 C 3 to Karl Heinz Hesse et al. teaches a switching circuit for periodical testing of the functioning of parts of an anti-skid protected, pressure-actuated motor vehicle brake system. The system employs several safety and automatic control circuits to obtain a test result.
The German Patent Laid Out 1,065,460 to Wilfried Fritzsche teaches a circuit for the switching on and switching off of inductance loads. The reference is very limited in that it does not address the particular problems associated with anti-skid systems.
The German Patent Application Laid Open DE-3,833,333 Al to Akihisa Nakamura teaches a control device for automatically controlling the throttle device of a combustion engine with internal combustion. The reference includes a control circuit with a cycle oscillating device for generating of a test signal of a time duration which is shorter than the reaction time of the throttle. It is submitted that this reference, while indicating certain control principles, clearly falls short to be of use in connection with an anti-skid system.
The German Patent Application Laid Open DE-3,706,393 Al to Stanislav Jeroncic teaches the application of a internal-diagnosis method. The reference is in particular concerned with the measurement of a voltage drop along a strategically placed resistor. However, the reference does not concern a structure related to an anti-skid system.
The German Patent Application Laid Open DE-OS 3,701,795 Al to Masahiko Asakura teaches a detection method for abnormal behavior in an air/combustion fuel ratio control system for combustion engines. While the reference is concerned with the detection of certain ratios and employs a computer logic and control circuits, it clearly does not provide a circuit directly suitable for an anti-skid system.
The German Patent Application Laid Open DE-OS 3,701,714 Al to Wolfgang Drobny et al. teaches a method and a device for surveillance of computer-controlled set members. The reference is directed to determining certain error cases based on signal probes. However, no system is taught which would be directly applicable to an anti-skid safety system.
The German Patent Application Laid Open DE-OS 3,522,392 Al to Fritz Hofmann et al. teaches an electric set system, in particular for operating a steering gear. The output of the final amplifier 8 or, respectively, of the set motor 2 of the reference structure is connected to a test circuit 17, which tests the operation of the set motor within the framework of a pulsed test operation under reduced load. It is noted that this reference is not concerned with an anti-skid system and, in particular, with safety features required and appropriate for such a system.
The German Patent Application Laid Open DE-OS 3,447,449 Al to Peter Cyril Byrne teaches an electronic control apparatus for an anti-skid system. A microcomputer is employed and the monitoring circuit comprises a reset device for the microcomputer, which resets the microcomputer if the performance of the program falls out of synchronization. The sensing device tests if the wheel speed sensors are short-circuited or separated or not. It appears that, testing of the sensors, however, will not assure that a braking effect will occur even if the sensors operate.
The German Patent Application Laid Open DE-OS 3,446,645 Al to Karl Kapp teaches a switching arrangement for the formation of a current signal. According to the reference, the user load 5 is connected diagonally relative to a transistor bridge 1-4, and a supply voltage source is connected to a second diagonal of the transistor bridge. While the reference further teaches that the current signal can be picked up at two corresponding transistors, nevertheless, this system shows very little which would indicate any applicability in connection with a braking system with anti-skid protection.
The German Patent Application Laid Open DE-OS 3,322,074 Al to Wolfgang Kosak teaches an emergency running device for microcomputer-controlled systems. An emergencyrun function generator 24 is provided and, in case of the occurrence of a fail-safe signal U.sub.FS, actuates a reset input 21 of a microcomputer 10 and, simultaneously, feeds the emergency signal U.sub.N via a logic 30 to the system. While these features and structures may have certain merits in connection with other devices, it is not clear what the structure, taught in this reference, would do and if it would be suitable to be used in connection with an anti-skid protection system.
The German Patent Application Laid Open DE-OS 3,227,285 Al to Shinichi Hori teaches an automatic control system for drive machines. The control system of this reference is formed such that, in case of the expected occurrence of an error in the mode of operation of the set actuation switch, the switch remains out of operation and, in case of a standard control by the set actuation switch, indicates the operational state of the system. No specific relation appears to be made in this reference to the exigencies and requirements of an anti-skid system.
The German Patent Application Laid Open DE-OS 2,928,981 to Karl-Heinz Hesse et al. teaches a test circuit for the controller of an anti-lock protected motor vehicle brake, which is furnished with at least one control microcomputer for the control channels. It appears, however, that this reference does not specifically relate to a control of the actual brake function, but relates to a testing of the automatic control device.
The German Patent Application Laid Open DE-OS 2,909,384 to Takao Sato teaches an automatic control arrangement for the control rod drive of hot-water reactors. The reference surveys the operation of a respective step motor with a test signal. It is believed that this teaching does not teach how to meet the requirements of an anti-skid protection system in a motor vehicle.
The German Democratic Republic Patent 146,658 to Gerd Niemand teaches a measurement and surveillance circuit for dynamic electrical characteristic values. The voltage furnished by a measurement probe is compared to two reference values and is processed by way of comparators, trigger circuits, and an integrator circuit. No direction is given in this reference to employ any of the circuitry illustrated in connection with anti-skid protection system in a vehicle.
The U.S. Pat. No. 4,635,607 to Yasuoka et al. teaches an apparatus for controlling the supply of fuel to an internal combustion engine. The detector of the reference is to detect any disorder in the amount of combustion air and for providing a disorder indicating signal. However, the presence of the amount of combustion air appears to be an insufficient detection basis where an electric anti-skid protection system is desired to be automatically controlled.
The U.S. Pat. No. 4,547,740 to Kurt A. I. Nilsson teaches a monitoring device for integrated drive amplifiers which is intended to be driven by digital signals and which includes fault detection circuits. However, this circuit structure does not appear to be directly concerned skid protection system for a vehicle.
The Patent Abstract of Japan, entitled "Overload Detecting Circuit of Power Amplifier," of May 18, 1979, Vol. 3, No. 58, applied for in Japan on Aug. 29, 1977 and receiving the Japanese Publication No. 54-37448, is concerned with the existence of certain overload conditions. It appears, however, that this reference does not pay particular attention to the specific needs existing in connection with an anti-skid protection system for a motor vehicle.